My dog shucked his Mountain Pack

This pack is great. My dog is an Aussie Blue heeler mix and he loves hiking and camping. The pack I bought was a large, which was too large for him. But he did well with it, took a month to teach him not to drop his head and shuck the pack. the support system could have a way to stop the pack shuck. Any way the last time we went on a long hike he was carrying his food and half our water when he went off trail after a squeral &lt;(spell check) and returned without his pack and no rodent either. So I'm gonna buy another smaller pack this time. Rich Hill and Archer